1. Field of the Invention
The present invention relates to an abnormality detecting device for an automobile engine capable of detecting a misfire or abnormality in an air-fuel ratio in the engine to thereby generate an alarm.
2. Discussion of Background
There have been proposed to control precisely an air-fuel ratio in an automobile engine and to use a three component catalyst in order to purify harmful gas discharged from the engine. Further, there has been increasing such demand that when the quality of exhaust gas becomes worse from the initial value by any cause, detection of such state is made; an alarm is generated, and repairing is carried out to return into a normal state.
A conventional technique which was proposed to meet such requirements will be described.
FIG. 6 is a diagram showing an abnormality detecting device for an automobile engine to detect abnormality in exhaust gas discharged from an automobile engine. In FIG. 6, reference numeral 1 designates an engine, numeral 2 designates an intake air pipe, numeral 3 designates an injector for injecting fuel to the intake air pipe 2, numeral 4 designates an airflow sensor for measuring a load to the engine (e.g. an intake air quantity), numeral 5 designates a revolution speed sensor for generating pulses in response to a revolution speed of the engine, numeral 6 designates an exhaust pipe, numeral 7 designates a catalyst to purify exhaust gas, numeral 8 designates an oxygen sensor to detect a component of the exhaust gas, and numeral 9 designates a controlling device which operates a required fuel quantity on the basis of various input data to thereby control the pulse width of a driving signal to the injector 3, and performs abnormality judgment to a fuel supply system whereby an alarm lamp 10 is operated when the fuel supply system is detected to be abnormal.
The operation of the conventional abnormality detecting device will be described. Information signals from the airflow sensor 4 and the revolution speed sensor 5 are inputted as major parameters to the controlling device 5 where a basic fuel quantity is calculated. Further, the basic fuel quantity is corrected depending on a rich or lean state of the exhaust gas, which is indicated by the oxygen sensor 8. The detail explanation of the above mentioned operations is omitted since they are well-known techniques.
Now, detection of abnormality in the engine, which relates to the present invention, will be described in detail. The airflow sensor 4 is used as a detecting means to detect a load to the engine. In order to detect a load to the engine, another information such as a negative pressure in the intake air pipe or a throttle valve opening degree may be used.
The present invention concerns the detection of an abnormal state occurring in an automobile engine, particularly, the detection of abnormality in a fuel supply system or an ignition system for the engine, more specifically, the detection of a misfire or abnormality in an air-fuel ratio which deteriorates the quality of exhaust gas discharged from the engine.
FIG. 7 is a flowchart showing an example of the detection of a misfire. A pulse signal outputted from the revolution speed sensor 5 is received in the controlling device 9 at step S1. A period of pulses is measured from the input signal at step S2. The presence or absence of a misfire is judged on the basis of values which indicate changes of angular speed at step S3.
As shown in FIG. 8a which shows a period of pulses in the signal of the revolution speed sensor 5 in a normal state, when there is no misfire, a torque of each of cylinders #1, #2, #3 and #4 has the same level, whereby each period assumes substantially the same value. However, when a misfire takes place, for instance, at the third cylinder #3 due to the stained spark plug or a defect in the ignition coil or the high voltage cord, the third cylinder #3 doesn't produce any torque. Accordingly, the period of the pulses becomes irregular as shown in FIG. 8b, whereby it is possible to detect the occurrence of a misfire. The measurement of the period of the pulses and the judgment of misfiring are carried out by a microcomputer (not shown) in the controlling device 9.
FIG. 9 is a flowchart showing an example of processes to detect abnormality in an air-fuel ratio. The concentration of oxygen contained in exhaust gas is detected with use of the oxygen sensor 8 at step S4; the controlling device 9 performs a feedback control for the air-fuel ratio in response to a detection quantity of the oxygen concentration by the oxygen sensor 8, whereby a feedback correction quantity is calculated at step S6.
FIG. 10a is a diagram showing an output signal from the oxygen sensor 8. FIG. 10b is a diagram showing an air-fuel ratio correction quantity in a normal operating state. When the air-fuel ratio deviates from a regular range of value due to an abnormal state of the airflow sensor 4, the injector 3 or the controlling device 9, the air-fuel ratio correction quantity assumes a state in which the average value is in an off-set state (Ce) as shown in FIG. 10c, which shows a large value in comparison with a value: Ce.perspectiveto.0 which is in a normal state as shown in FIG. 10b.
When the off-set value Ce exceeds a predetermined value (e.g. 30%), a judgment that abnormality takes place in the fuel supply system is made by the controlling device 9 at step S7.
As described above, according to the conventional abnormality detection device, a misfire due to a faulty operation of the spark plug or the ignition coil in the ignition system or abnormality in air-fuel ratio which is resulted from an abnormal control of the injector due to a faulty operation of the airflow sensor 4, the injector 3 or the control device 9 are detected, and an alarm is generated whereby a driver can recognize that the exhaust gas from the engine becomes deteriorated. Then, the driver can correct the abnormal state into a normal state.
In the conventional abnormality detecting device for an automobile engine having the above-mentioned construction, even when an abnormal state does not take place in a structural component such as the airflow sensor 4, the injector 3, the spark plug, the ignition coil and so on, air bubbles are contained in the fuel ejected from the injector 3 in a case that supply of fuel becomes temporarily short. As a concrete example, it occurs when the engine is in a high temperature state after the engine has been stopped and again started. As a result, fuel to be supplied becomes short, and the air-fuel ratio exhibits a substantially lean state. In this case, judgment of misfire or abnormality in air-fuel ratio is made. Since the above-mentioned phenomenon is not abnormality in the component but a temporary shortage of fuel after the re-starting of the engine, it is improper to provide an alarm to the driver.